1. Field of the Invention
The present invention relates to an optical pickup device comprising at least: an objective lens whose numerical aperture (NA) is not less than 0.75; and a phase shift element which corrects a spherical aberration generated due to a difference in substrate thickness between first and second optical recording mediums when the objective lens is used in cases where the first and second optical recording mediums having different substrate thicknesses are selectively recorded or reproduced by using first and second laser lights having different wavelengths.
2. Description of the Related Art
In general, optical recording mediums such as a discoid optical disc or a card-shaped optical card are often used since they can record information signals of, e.g., video information, sound information or computer data on tracks spirally or concentrically formed on a transparent substrate with a high density, and access a desired track at a high speed when reproducing recorded tracks.
Although an optical disc which serves as this type of optical recording medium, e.g., a DVD (Digital Versatile Disc) or the like has been already commercially available, a Blu-ray Disc which can record or reproduce information signals with a very higher density than the DVD has been vigorously developed.
The above-described DVD records or reproduces information signals on a signal surface placed at a position apart from a laser beam incidence surface by approximately 0.6 mm by applying a laser beam obtained by narrowing down a laser light whose wavelength is approximately 650 mm by an objective lens whose numerical aperture (NA) is approximately 0.6. At this time, a recording capacity of the DVD is approximately 4.7 GB (gigabytes) on one side when a diameter of a disc substrate is 12 cm.
On the other hand, the above-described Blu-ray Disc has been developed so that it can record or reproduce information signals on a signal surface placed at a position apart from a laser beam incidence surface by approximately 0.1 mm by applying a laser beam obtained by narrowing down a laser light whose wavelength is not more than 450 nm by an objective lens whose numerical aperture (NA) is not less than 0.75. At this time, a recording capacity of the Blu-ray Disc is approximately 25 GB (gigabytes) on one side when a diameter of a disc substrate is 12 cm.
Meanwhile, with advance of the development of the Blu-ray Disc, there has been developed an optical pickup device which can perform recording or reproduction while assuring downward compatibility between the Blu-ray Disc whose recording density is an extra-high density and the DVD whose recording density is lower than that of the Blu-ray Disc by using one objective lens (e.g., Japanese Patent Application Laid-open No. 2002-236253 (pp. 57-58, FIG. 31), and Phase Shift Element for Blu-ray Disc/DVD Compatibility, Katsuhiro Koike et., al., Technical digest for ODS 2003, WA6).
Further, an optical pickup device which can correct a chromatic aberration with respect to the Blu-ray Disc has been developed (e.g., Japanese Patent Application Laid-open No. 2003-272213 (pp. 5-6, FIG. 2), and Japanese Patent Application Laid-open No. 2003-270525 (p. 6, FIG. 3)).
Furthermore, an optical pickup device which can correct a chromatic aberration with respect to a general optical disc has been also developed (e.g., Japanese Patent Application Laid-open No. hei6(1994)-82725 (p. 2, FIG. 1)).
FIG. 1 is a view schematically showing an optical system of an optical pickup device according to Conventional Example 1. FIG. 2 is a view schematically showing an optical system of an optical pickup device according to Conventional Example 2. FIG. 3 is a view schematically showing an optical system of an optical pickup device according to Conventional Example 3. FIG. 4 is a view schematically showing an optical system of an optical pickup device according to Conventional Example 4. FIG. 5 is a view schematically showing an optical system of an optical pickup device according to Conventional Example 5.
First, an optical pickup device 110 according to Conventional Example 1 shown in FIG. 1 is described in the Japanese Patent Application Laid-Open No. 2000-236253. The device will be briefly described with reference to the document. The optical pickup device 110 according to Conventional Example 1 is configured so that a first optical disc 101 having a transparent substrate whose thickness is 0.1 mm (e.g., a next-generation high-density optical disc using a blue laser) and a second optical disc 102 having a transparent substrate whose thickness is 0.6 mm (e.g., a DVD) can be selectively applied.
The optical pickup device 110 according to Conventional Example 1 comprises: a first semiconductor laser 111 which emits a first laser light (a blue laser light) having a wavelength of approximately 400 nm in accordance with the first optical disc (e.g., a next-generation high-density optical disc) 101; a second semiconductor laser 112 which emits a second laser light (a red laser light) having a wavelength of approximately 650 nm in accordance with the second optical disc (e.g., a DVD); first and second beam splitters 113 and 114; a collimator lens 116 which is movable in an optical axis direction by a one-dimensional actuator 115; a ¼ wave plate 117; an aperture 118; an objective lens 120 which has a numerical aperture NA of 0.7 or above in order to form images of the first and second laser lights on the first and second optical discs by a two-dimensional actuator 119 and has a diffraction annular lens formed on at least one surface; and a cylindrical lens 121 and a photodetector 122 which detect return lights from the first and second discs 101 and 102.
Moreover, respective divergent light beams exiting from the first and second semiconductor lasers 111 and 112 are selectively condensed on information recording surfaces 101a and 102a of the first and second optical discs 101 and 102 through the first and second beam splitters 113 and 114, the collimator lens 116 and the ¼ wave plate 117 and the aperture 118, thereby forming respective spots. In this example, in cases where there are errors in substrate thicknesses of the first and second optical disc, where there are errors in respective oscillation wavelengths due to manufacture errors of the first and second semiconductor lasers 111 and 112, or where there are errors in thicknesses of the lenses constituting the converging optical system, a generated spherical aberration is corrected by movement of the collimator lens 116.
Additionally, since the objective lens 120 converges a light beam from the first semiconductor laser 111 within a diffraction limit in an image side numerical aperture NA1, information recorded on the first optical disc 101 at a high density can be reproduced. On the other hand, since the objective lens 120 converges a light beam from the second semiconductor laser 112 within a diffraction limit in an image side numerical aperture NA2, information recorded on the second optical disc 102 can be reproduced. Further, when converging a light beam from the second semiconductor laser 112 on the information recording surface 102a of the second optical disc 102, since a light beam which passes through an area from the image side numerical aperture NA1 to the counterpart NA2 is formed as a flare component by an effect of the diffraction annular lens formed on at least one surface of the objective lens 120, the light beam passing through the area from the image side numerical aperture NA1 to the counterpart NA2 does not form a spot on the information recording surface 102a of the second optical disc 102 even if the whole light beam from the second semiconductor laser 112 is caused to pass through the aperture 118 determined by NA1. Therefore, aperture switching means for NA1 and NA2 does not have to be provided.
An optical pickup device 130 according to Conventional Example 2 shown in FIG. 2 is disclosed in the above reference by Katsuhiro Koike et. al. Giving a brief description in conjunction with the reference by Katsuhiro Koike et. al., in the optical pickup device 130 according to Conventional Example 2, a phase shift element (PSE) 132 and an objective lens 133 whose numerical aperture (NA) is 0.85 are attached in a lens holder 131, a Blu-ray Disc compatible with a wavelength of 405 nm and a DVD compatible with a wavelength of 650 nm can be selectively applied, and a spherical aberration generated due to a difference in substrate thickness between the Blu-ray Disc and the DVD can be corrected by the phase shift element 132.
In this example, the phase shift element 132 (PSE) has a tiered phase difference pattern portion 132a formed on an inner portion thereof, and a flat portion 132b is formed on a circular portion around the tiered phase difference pattern portion 132a. 
A first laser light having a wavelength of 405 nm is transmitted through the tiered phase difference pattern portion 132a and the flat portion 132b of the phase shift element 132 as it is with respect to the Blu-ray Disc so that the first laser light is condensed on the Blu-ray Disc. On the other hand, a second laser light having a wavelength of 650 nm is transmitted through the tiered phase difference pattern portion 132a only of the phase shift element 132 with respect to the DVD so that the second laser light is condensed on the DVD by using the objective lens 133 while a spherical aberration is corrected by the tiered phase difference pattern portion 132a. 
An optical pickup device 140 according to Conventional Example 3 shown in FIG. 3 is a device disclosed in Japanese Patent Application Laid-open No. 2003-272213. Giving a brief description in conjunction with this publication, the optical pickup device 140 according to Conventional Example 3 comprises: a beam expander 141 comprising a concave lens 141A and a convex lens 141B; a triplet 142 which is formed by attaching a concave lens 142A, a convex lens 142B and a convex lens 142C and serves as chromatic aberration correcting means; and an objective lens 143 whose numerical aperture is not less than 0.7. This optical pickup device 140 can correct a spherical aberration and a chromatic aberration with respect to a Blu-ray Disc 101 for a laser light L whose wavelength is approximately 403 nm.
In this example, the beam expander 141 changes the parallelism of a light by adjusting a gap between the two lenses 141A and 141B, thereby correcting a spherical aberration of the objective lens 143. Furthermore, the triplet 142 corrects an error component in a focal direction generated by a chromatic aberration of the objective lens 143.
An optical pickup device 150 according to Conventional Example 4 shown in FIG. 4 is disclosed in Japanese Patent Application Laid-open No. 2003-270525. Giving a brief description in conjunction with this publication, the optical pickup device 150 according to Conventional Example 4 comprises: a beam expander 151 comprising a concave lens 151 and a Fresnel lens 151B; and an objective lens 152 whose numerical aperture is not less than 0.7, and can correct a spherical aberration and a chromatic aberration with respect to a Blu-ray Disc 101 for a laser light L whose wavelength is approximately 405 nm.
In this example, the beam expander 151 changes the parallelism of a light by adjusting a gap between the two lenses 151A and 151B, thereby correcting a spherical aberration of the objective lens 152. Furthermore, an analog blaze 151Ba (or a tiered blaze) is formed to the Fresnel lens 151B in the beam expander 151, and functions as a convex lens with respect to the concave lens 151A. A focal distance of the Fresnel lens 151B is set so that an error component in a focal direction generated by a chromatic aberration of the objective lens 152 can be corrected.
An optical pickup device 160 according to Conventional Example 5 shown in FIG. 5 is disclosed in Japanese Patent Application Laid-open No. 82725-1994. Giving a brief description in conjunction with this publication, the optical pickup device 160 according to Conventional Example 5 comprises: a chromatic aberration correction element 161 whose flat surface vertical to an optical axis is formed as a concentric annular zone having a tiered shape on at least one of a light incidence end surface 161a and a light projection end surface 161b; and an objective lens 162, and can correct a chromatic aberration with respect to an optical disc 103 by using the single chromatic aberration correction element 161.
Meanwhile, in the optical pickup device 110 according to Conventional Example 1, the first optical disc 101 having a transparent substrate whose thickness is 0.1 mm and the second optical disc 102 having a transparent substrate whose thickness is 0.6 mm can be selectively applied by the objective lens 120 whose numerical aperture NA is not less than 0.7 and has the diffraction annular lens formed on at least one surface, but a pitch of the diffraction annular lens formed on at least one surface of the objective lens 120 is narrow, and machining of the objective lens 120 is hard, which may possibly adversely affect the lens performance.
Furthermore, in the optical pickup device 130 according to Conventional Example 2, although the Blu-ray Disc compatible with a wavelength of 405 nm and the DVD compatible with a wavelength of 650 nm can be selectively applied by the phase shift element 132 and the objective lens 133 whose numerical aperture NA is 0.85, a spherical aberration is corrected with respect to the second laser light by the tiered phase difference pattern portion 132a formed on the inner portion of the phase shift element 132. However, in regard to the first laser light which is transmitted through the tiered phase difference pattern portion 132a formed on the inner portion and the flat portion 132b formed on the outer circular portion as it is, if a wavelength error is generated, the correction of a spherical aberration with respect to the Blu-ray Disc becomes lax since the outer circular portion is flat.
Moreover, in the optical pickup device 140 according to Conventional Example 3, although a spherical aberration and a chromatic aberration can be corrected with respect to the Blu-ray Disc 101 by the beam expander 141, the triplet 142 and the objective lens 143 whose numerical aperture is not less than 0.7, assuring the downward compatibility between the Blu-ray Disc 101 and the DVD (not shown) to be recorded or reproduced is not considered. Additionally, since the triplet 142 which serves as the chromatic aberration correcting means must be designed in such a manner that it can correct an epaxial chromatic aberration excessively in the entire optics, a curvature radius of the attached surface becomes small, and machining is difficult. Furthermore, in cases where a spherical aberration is corrected by the beam expander 141, changing a gap in the beam expander 141 can suffice, but a time required to correct the spherical aberration becomes long.
Moreover, in the optical pickup device 150 according to Conventional Example 4, since a spherical aberration and a chromatic aberration can be corrected by the beam expander 151 only which has the blaze, the equivalent performance can be obtained with respect to the Blu-ray Disc 101 even if the number of components is reduced to be less than those in Conventional Example 3. However, assuring the downward compatibility between the Blu-ray Disc 101 and the DVD (not shown) to be recorded or reproduced is not considered. Additionally, when the analog blaze 151Ba (or the tiered blaze) is formed to the Fresnel lens 151B in the beam expander 151, a pitch becomes narrow, and machining is thereby difficult.
Further, in the optical pickup device 160 according to Conventional Example 5, since a chromatic aberration can be corrected with respect to the optical disc 103 by the chromatic aberration correction element 161 formed into a tiered shape, this device can be likewise applied to the extra-high density Blu-ray Disc, but assuring the downward compatibility between the Blu-ray Disc and the DVD to be recorded or reproduced is not considered.